An Iraqi criminal prisoner was tortured and beaten to death within three days of being turned over to police in Basra by British troops.

This latest detailed evidence of previously covered-up Iraq atrocities has emerged following the leak of a vast number of Iraq war logs compiled by the US army and containing hour-by-hour military field reports.

The 391,832 previously secret field reports, passed to the Guardian and other newspapers by the online whistleblowing group WikiLeaks, has already shown that US authorities failed to investigate hundreds of reports of abuse, rape and murder by Iraqi police and soldiers.

According to the new evidence, British authorities were well aware of the atrocities that were occurring in Basra and were unhappy about them.

An autopsy was conducted on the prisoner, the police officer said to have killed him was named in a UK investigation, and high-level diplomatic protests were made to the Iraqi interior minister, without apparent result.

Documents obtained by the Guardian and by a Danish newspaper detail the arrest of Abbas Alawi by a joint British and Danish patrol on 10 April 2005.

In Operation Grey Wolf, Alawi – code-named Bravo One – was picked up along with three other men in a dawn raid by British infantry of the Duke of Wellington's Regiment. The logs record that the prisoners were handed over to the Basra police, controlled by Iraq's ministry of the interior.

A press officer boasted on the Danish military website at the time that Alawi was a much-feared gangster and fuel smuggler who had terrorised his neighbourhood.

A Danish lawyer, Jes Rynkeby Knudsen, currently serving as a military judge advocate, confirmed today that Alawi was killed by the police. He told the Danish newspaper Dagbladet Information: "I saw pictures of his body afterwards."

Alawi was kept in a secret and illegal jail for three days, from 10-13 April. When his blood-covered corpse was collected by relatives, the Basra governor ordered an autopsy which "substantiated charges that Allawi was beaten to death".

The death appears to have been a last straw for the exasperated western occupiers. On 1 June the Danish ambassador, Torben Getterman, accompanied by a British diplomat, Tim Torlot, and the US charge d'affaires, David Satterfield, confronted the Iraqi interior minister in Baghdad.

According to documents from diplomatic sources, they told the minister, Baqr Jabr, who was also responsible for the notorious Wolf Brigade torture squads, that the situation caused "grave concern". The diplomatic trio handed over an intelligence report of an investigation carried out by the British 12th Mechanised Brigade HQ.

It implicated three members of the investigations support unit (ISU), including a named lieutenant said to have killed Alawi, in "assassinations, kidnappings, intimidation, threats against the judiciary and illegal attacks against the citizens".

Torlot told the minister: "The problem … is far more widespread than indicated in a single report. Grave concerns regarding the Basra situation extend to the ministry of the interior as a whole … The crimes of the corrupt Iraqi police are well known on the streets of Basra."

A few days earlier, the Basra chief of police, General Hassan al-Sade, who was advised by a British deputy chief constable, Colin Smith, admitted to the Guardian that the Basra police were effectively out of control. He said: "I trust 25% of my force, no more."

The Iraq war logs have provided a wealth of evidence about the reported abuse of detainees by Iraqi forces, with at least six people reported to have died. US army reports, some supported by medical evidence, describe prisoners experiencing whipping, kicks, punches or electric shocks. As recently as December US forces were handed a video seemingly showing Iraqi army officers executing a prisoner.

Other revelations from the files include an incident in 2007 in which a US helicopter gunship killed two Iraqi men attempting to surrender and a previously unknown US military tally of civilian and insurgent casualties during the US occupation. According to Iraq Body Count, which monitors civilian casualties in the country, the leaked files detail around 15,000 extra civilian deaths.

A US intelligence analyst formerly based in Baghdad, Bradley Manning, faces a court martial charged with leaking similar material to WikiLeaks, which has also released similar logs concerning operations in Afghanistan.